Pulping. The transition of a tree into paper involves several discrete stages. Stage one is the debarking of the tree and the conversion of the tree into wood chips. Stage two is the conversion of wood chips into pulp. This conversion may be by either mechanical or chemical means.
Bleaching is the third stage. Delignification is the first step in the bleaching of chemical pulps. Lignin, a complex polymer derived from aromatic alcohols, is one of the main constituents of wood. During the early stages of bleaching, residual lignin, which constitutes 3-6% of the pulp, is removed. Currently, this is typically done by treatment of the pulp with elemental chlorine at low pH, followed by extraction with hot alkali. Once a significant portion of the residual lignin has been removed, the pulp may be whitened, by a variety of means, to high brightness. Chlorine dioxide is commonly used in the brightening step.
Although chlorine compounds are effective and relatively inexpensive, their use in pulp mills results in the generation and release of chlorinated organic materials, including dioxins, into rivers and streams. Due to increasing regulatory pressures and consumer demand, new, non-chlorine bleaching technologies are urgently needed by manufacturers of paper-grade chemical pulps.
In recent years, attention has been drawn to the potential use of enzymatic processes associated with fungal degradation of lignin to develop environmentally friendly technologies for the pulp and paper industry. In many wood-rotting fungi, extracellular metalloenzymes such as glyoxal oxidase, a copper-containing oxidase, in combination with lignin and manganese peroxidases, both of which contain iron in a protoheme active site, harness the oxidative capability of dioxygen and direct its reactivity to the degradation of lignin within the fiber walls. In this biochemical process, high valent transition metal ions serve as conduits for the flux of electrons from lignin to dioxygen.
Therefore, transition metal ions are known to possess redox properties that are useful in the delignification and bleaching of lignocellulosic materials. However, the behavior of transition metal ions in water is often difficult to control. In aqueous solution, complex equilibria are established between ionic hydroxides and hydrates, as well as between accessible oxidation states of the metal ions. In addition, many transition metal oxides and hydroxides have limited solubilities in water, where the active metals are rapidly lost from solution as solid precipitates. What is needed in the art of pulp bleaching is a reusable transition metal-derived bleaching agent composed of relatively inexpensive and non-toxic materials that is suitable for use in a bleaching procedure.
Polyoxometalates. Polyoxometalates are discrete polymeric structures that form spontaneously when simple oxides of vanadium, niobium, tantalum, molybdenum or tungsten are combined under the appropriate conditions in water (Pope, M. T. Heteropoly and Isopoly Oxometalates Springer-Verlag, Berlin, 1983). In a great majority of polyoxometalates, the transition metals are in the d.sup.o electronic configuration which dictates both high resistance to oxidative degradation and an ability to oxidize other materials such as lignin. The principal transition metal ions that form polyoxometalates are tungsten(VI), molybdenum(VI), vanadium(V), niobium(V) and tantalum(V).
Isopolyoxometalates, the simplest of the polyoxometalates, are binary oxides of the formula [M.sub.m O.sub.y ].sup.p-, where m may vary from two to over 30. For example, if m=2 and M=Mo, then the formula is [Mo.sub.2 O.sub.7 ].sup.2- ; if m=6, then [Mo.sub.6 O.sub.19 ].sup.2- ; and if m=36, then [Mo.sub.36 O.sub.112 ].sup.8-. Polyoxometalates, in either acid or salt forms, are water soluble and highly resistant to oxidative degradation.
Heteropolyoxometalates have the general formula [X.sub.x M.sub.m O.sub.y ].sup.p- and possess a heteroatom, X, at their center. For example, in the .alpha.-Keggin structure, .alpha.-[PW.sub.12 O.sub.40 ].sup.3-, X is a phosphorus atom. The central phosphorus atom is surrounded by twelve WO.sub.6 octahedra.
Removal of a (M=O).sup.4+ - moiety from the surface of the .alpha.-Keggin structure .alpha.-[PM.sub.12 O.sub.40 ].sup.3-, where M is molybdenum or tungsten, creates the "lacunary" .alpha.-Keggin anion, .alpha.-[PM.sub.11 O.sub.39 ].sup.7-. The lacunary .alpha.-Keggin ion acts as a pentadentate ligand for redox active transition metal ions, such as vanadium(V) in .alpha.-[PVW.sub.11 O.sub.40 ].sup.4-. Further substitution is also possible, giving anions of the form [X.sub.x M'.sub.m,M.sub.m O.sub.y ].sup.p-, such as [PV.sub.2 Mo.sub.10 O.sub.40 ].sup.5-. The redox active metal ions are bound at the surface of the heteropolyanions in much the same way that ferric ions are held within the active sites of lignin or manganese peroxidases. However, while stabilizing the metal ions in solution and controlling their reactivity, the heteropolyanions, unlike enzymes or synthetic porphyrins, are highly resistant to oxidative degradation (Hill, et al., J. Am. Chem. Soc. 108:536-538, 1986).
Previously, polyoxometalates have been used as catalysts for oxidation under heterogeneous and homogeneous conditions, analytical stains for biological samples, and for other uses still in development. The use of polyoxometalates in pulp bleaching has neither been described nor suggested.